in the river last but several hours and then subside rapidly. The river swells only at times of general rain, and it then covers the whole flood plain over a width of a quarter of a mile.
Along the line of a single valley are strung the scattered plots of precious watered land. Between them there may be nothing but gravel-strewn stretches of valley flat. Hence it is
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Fig. 77—Pingo-pingo, a wild desert shrub common in the southern part of the Desert of Atacama. The photograph was taken near San Pedro de Atacama.
natural that each cultivated tract should be known under a different name and give its name to a part of the valley. The best illustration is to be found in the valley of Rio Atacama. The town of San Pedro de Atacama has about 500 people, but all about it (and to the inexperienced traveler they appear a part of it) are scattered groups of families and little villages. All told they raise the population of the district to 2000. On the maps the name of San Pedro de Atacama is applied to the whole collection of groups. Each cultivated district, or aillo, has a distinct name, and this name is given to the central pueblo or village as well, the use of the word aillo apparently being interchangeable as between Jand and group. Even the largest village of the group is not called San Pedro but Conde Duque. For two leagues above this nucleus are small culti-
